Process for making high moisture barrier polyvinyl chloride film

ABSTRACT

A process for making a self-supporting, plasticized, uncoated polyvinyl chloride film wherein a melt containing polyvinyl chloride resin and additives, including a resin plasticizer and from about 0.5 to 4.0 parts per hundred based upon the weight of the polyvinyl chloride resin of a moistureproofing paraffin wax, is extruded as a continuous film. Such film may then be elongated while within an orientation temperature range and then cooled to a temperature below such orientation temperature range while held under tension.

This application is a division of application Ser. No. 359,774, filedMay 14, 1973, now U.S. Pat. No. 3,875,164.

Commercially available polyvinyl chloride films generally exhibit a highdegree of flexibility, transparency and gloss, an attractive appearance,and good elongation and tenacity characteristics, and are therefore welladapted for many packaging applications. However, lacking high moisturebarrier characteristics, such films have found little use in packagingbaked goods and sweet doughs, candy, frozen foods and other productswhich suffer in quality and/or appearance when exposed to an excess orexperience a loss of moisture. Notwithstanding the attention received,this deficiency has not been remedied without some sacrifice in otherdesirable film properties and/or without resorting to complicated andexpensive production procedures.

Solvent coatings containing paraffin wax are employed inmoistureproofing regenerated cellulose films and it might be expectedthat, by this technique or by the use of formulations containing suchwax, moistureproof polyvinyl chloride films could also be obtained. Thisexpectation has been dispelled by U.S. Pat. No. 2,435,464 which explainsthat solvent coatings are inapplicable to vinyl films, and that filmsprepared from vinyl resin formulations containing varying proportions ofdifferent paraffin waxes exhibited substantially no improved moistureresistance.

Mentioned in this patent is that somewhat better results were securedwhen the waxes were blended into vinyl formulations by means of a rosinbut that this technique must be carefully carried out and theproportions of the ingredients must be adjusted in accordance with thethickness of the film proposed to be made. Absent, however, is anyindication of the blending techniques, the proportions of ingredients,the process by which the films were made or the results which wereachieved.

U.S. Pat. No. 2,445,727 not only confirms that polyvinyl chloride filmscontaining paraffin waxes show no improvement in moisture transmissionresistance but that, in some cases, the presence of such waxes wasactually a detriment with respect to this particular characteristic.This patent does disclose that vinyl formulations, plasticized withbutadieneacrylonitrile type elastomers and containing paraffin waxes,may be cast into clear films. However, mixtures of solvents or solventsand resinous gums are required, and satisfactory films can be made onlyby utilizing expensive solution casting techniques.

A primary object of this invention is the provision of a new or improvedand more satisfactory process for making a polyvinyl chloride filmhaving high resistance to moisture transmission.

Another object of this invention is a process for preparing aplasticized, uncoated, paraffin wax-containing polyvinyl chloride filmhaving desirable characteristics comparable to conventional polyvinylchloride films and which further possess a high degree of resistance tomoisture transmission.

Still another object of this invention is to provide a process formaking an extruded, plasticized, uncoated polyvinyl chloride film havinghigh moisture barrier properties, particularly after being annealed.

A further object is the provision of a process for making an extruded,uncoated, annealed polyvinyl chloride film having high moisture barrierproperties which is formed of a plasticized polyvinyl chlorideformulation containing a paraffin wax.

A still further object is the provision of a process for makingplasticized, uncoated, annealed polyvinyl chloride films having highresistance to moisture permeability by economical extrusion techniquesand using conventional equipment.

In the drawings, FIGS. 1 and 2 are graphs illustrating the moisturevapor transmission characteristics of conventional polyvinyl chloridefilms and films formed in accordance with the process of the presentinvention.

These and other objects are accomplished in accordance with the presentinvention by an extrusion process in which a plasticized polyvinylchloride resin formulation containing from about 0.5 to 4.0 parts perhundred of a moistureproofing paraffin wax, based upon the weight ofpolyvinyl chloride present, is shaped into a continuous film. Such filmmay then be molecularly oriented and annealed. The shaped plasticized,uncoated polyvinyl chloride film exhibits a high resistance to moisturetransmission, which characteristics is enhanced as a result of theannealing step.

The plasticizer employed in the formulation may range from about 7 to 55parts per hundred, based upon the polyvinyl chloride present, andincludes both primary and secondary plasticizers in a ratio of from1.8/1 to 3.2/1. As plasticizers, the so-called "secondary plasticizers"are less efficient, yet as is generally known, such secondaryplasticizers do serve other functions, as for example, as hydrogenchloride scavengers, and thus are essential in the formulation forsatisfactory practice of the present invention.

Wax blending agents for assuring film clarity, particularly when highlevels of paraffin wax are used, heat stabilizers, extrusion lubricants,toners and the like may also be incorporated within the formulationwhere necessary or desired and have no apparent detrimental effect uponthe moisture barrier characteristic of the resulting film.

In the process of the present invention a formulation, containingpolyvinyl chloride resin, primary and secondary plasticizers, paraffinwax, a wax blending agent and any other necessary additives, is renderedmolten and extruded as a film through a suitable die. Preferably, atubular die is used with a gas under pressure being introduced into andcontained within the tubular film as it issues from the extrusion die toat least maintain the same in an expanded condition and, more desirably,to expand such film so as to thin the wall areas thereof which are stillat a temperature above its orientation temperature range.

After expansion, the tubular polyvinyl chloride film may be quenched,then reheated to a temperature within its orientation temperature rangeand stretched to orient the molecules thereof at least along one of itsaxial directions. The reheated portions of the tubular film may beoriented in a transverse direction by simply expanding the samelaterally under the pressure of the gas contained therein, and may beoriented longitudinally by being withdrawn from the location ofreheating at a faster rate than its delivery thereto.

The polyvinyl chloride film may be annealed immediately after itsorientation by heating the film to a temperature above its orientationtemperature range while maintaining such film under sufficient tensionas to permit, at most, only a minimal shrinkage thereof, and thereaftercooling the film to a temperature below its orientation temperaturerange while it is maintained in its tensioned condition. As will be moreapparent hereafter, annealing of the wax-containing polyvinyl chloridefilms provides for a drastic reduction in their moisture-vaportransmission characteristics. While a complete explanation for thisresult has not yet been established, it is believed that the heatapplied to the films during the annealing step may induce the containedwax to further blend and perhaps assume a layer formation of greatuniformity on the surface of such annealed film.

In the formulation employed in the present invention the quantitiesblended with a polyvinyl chloride resin are set forth in parts perhundred (phr), based upon the weight of the polyvinyl chloride resinpresent.

As referred to herein the "orientation temperature range" is the rangeof temperatures within which orientation of the polyvinyl chloridemolecules may be readily achieved by stretching, and extends from andgenerally slightly above the second order phase transition temperatureof the polyvinyl chloride resin to below a temperature at whichrelaxation of the orientation effect, which is achieved by stretching,occurs so rapidly that the film retains no significant orientation oncestretching ceases. While the orientation temperature range employed mayvary with different polyvinyl chloride formulations, temperatures offrom about 175° to 240° F are generally suitable.

Annealing or heat-setting of the oriented polyvinyl chloride film may beachieved by heating such film to a temperature within the range of fromabout 212° to 330° F while it is maintained under tension. Temperaturesbelow 212° F will generally not provide for the desired annealing, whiletemperatures approaching and exceeding 330° F necessitate extremecaution to avoid melting of the film. At practical commercial speeds,temperatures of from 240° to 270° F are most appropriate.

While less desirable, the oriented polyvinyl chloride film may beannealed after being snugly applied about an article which is to bepackaged. In this instance the article or its carrier must besufficiently rigid to permit only minimal change in the film dimensionsduring the heating and subsequent cooling of the film.

The duration of heat application during film annealing will vary withsuch factors as the temperatures employed, melting temperatures of thefilm, film orientation, etc., and may range from less than a second toperhaps 5 minutes or more.

Any commercial polyvinyl chloride resins of at least 90 mole percent ofvinyl chloride and having film-forming properties can be employed in theprocess of the present invention. Typical of such resins are "Exon"9269, manufactured by Firestone Chemical Company, "Escambia" 3250,manufactured by Escambia Chemical Corporation and DIAMOND 450, producedby Diamond Alkali Company. In addition to homopolymer polyvinyl chlorideresins, copolymers of vinyl chloride containing minor amounts of otherpolymerizable monomers, such as vinyl acetate, vinylidene chloride,acrylontrile, and the like, may be used in the formulation employed inproducing the film by the process of the present invention.

A variety of plasticizers may be used in the formulation including, asprimary plasticizers, di(2-ethyl-hexyl) adipate, di(2-ethyl-hexyl)phthalate, dibutyl phthalate, dibutyl sebecate, and, as secondaryplasticizers, epoxydized soy bean oil, such as, "Paraplex" G-62 suppliedby Rohm and Haas Company.

The total plasticizer content in the formulation may be varied fromabout 7 to about 55 parts per hundred, with a formulation containinglesser amounts of plasticizer, yet within the specified range, providinga film having reduced flexibility. As the total plasticizer content inthe formulation is reduced below 7 parts per hundred and less, the filmsbecome more rigid and exhibit poor physical characteristics, andparticularly tensile strength, and the formulation itself is difficultto extrude continuously. Incorporating more than about 55 parts perhundred total plasticizer in the formulation results in tacky filmshaving poor tensile properties.

With the total plasticizer maintained within the range specified,reducing the ratio of primary-to-secondary plasticizer in theformulation of less than 1.8 to 1 generally provides for unsatisfactoryfilms because of plasticizer exudation onto the film surfaces.Similarly, increasing the amount of primary plasticizer in theformulation to provide a primary-to-secondary plasticizer ratio whichexceeds a ratio of about 3.2 to 1 provide films which also areunsatisfactory, being extremely soft and tacky.

Of particular significance is that only moistureproofing paraffin waxes,such as, "Sunoco" 4412 and 5512 manufactured by the Sun Oil Company, aresuitable for use in providing the film of the present invention. Foundto be completely unsatisfactory are microcrystalline waxes ("Estawax" 20made by Petrolite Corporation), microcrystalline waxes containing 2%butyl rubber ("Multiwax" ML 445 made by Petroleum Specialties, Inc.),and high molecular weight polyethylene oxides ("Carbowax" 1000 and 6000produced by Union Carbide Corporation).

The paraffin wax employed preferably melt at a temperature ranging fromabout 125° to 200° F, and are incorporated into the formulation in anamount ranging between 0.5 to 3.0 to 4.0 parts per hundred and moredesirably from 1.0 to 3.0 parts per hundred. Less than about 0.5 partsper hundred of paraffin wax in the polyvinyl chloride formulationprovides for films having no significant decrease in moisture vaportransmission rate (MVTR), while the use of from 3.0 to 4.0 parts perhundred and more of the paraffin wax has an increasingly adverse effectupon the extrusion operation itself.

The presence of the paraffin wax in the formulation within the limitsspecified creates no unusual extrusion problems, nor do the resultingfilms exhibit tensile or elongation properties different from those ofconventional manufacture. Films formed from a formulation having from3.0 to 4.0 and more of the paraffin wax may exhibit some haze which canbe minimized, if not completely exhibited, by including in theformulation a wax blending agent, preferably in a 1 to 1 ratio with theparaffin wax. Satisfactory blending agents which are compatible with theother ingredients of the formulation include terpenes, polyterpenes andpolystyrenes, as for example, "Piccotex" 100, an alpha methylstyrene-vinyl toluene copolymer, produced by Penn Industrial ChemicalCompany.

Conventional heat-stabilizers may be used in the formulation, includingmetal stearates, such as calcium-zinc stearate ("MARK" QED produced byArgus Chemical Company) and bariumcadmium or dioctyl tins. A variety ofextrusion lubricants can be used, including stearic acid, low molecularweight polyethylene and glycerol monostearate. Commercially availabletoners, which are organic dyes on an aluminum hydroxide base, may alsobe added to the formulation to impart a desired color to the extrudedfilm.

For a better understanding of the present invention, reference is madeto the examples as hereinafter set forth. In all of the examples, thepolyvinyl chloride formulations were rendered molten and extruded ascontinuous tubular films using conventional equipment and well knownextrusion practices. A two stage screw type extruder having a three zonetemperature controlled barrel was employed, with typical extrusionconditions as follows:

    ______________________________________                                        Extruder barrel temperatures:                                                 ______________________________________                                         Zone 1               320° F                                            Zone 2               340° F                                            Zone 3               350° F                                           Die Adaptor           350° F                                           Extrusion Die         360° F                                           Screen Pack             80 to 40 mesh                                         Blow-up ratio of                                                               extruded tubular film - 3.5 to 1                                             ______________________________________                                    

After extrusion, expansion and quenching, the films were reheated andstretched 3.5 times their original dimensions to orient the same alongbiaxial directions. Immediately thereafter the films were annealed bybeing maintained at a temperature of about 250° F for a period of 10seconds while preventing any substantial change in its dimensions, andthen cooled to the temperature of the ambient atmosphere.

Moisture vapor transmission rates (MVTR) were determined by disposingthe sample films across the mouths of cups containing known weights ofcalcium chloride and subjecting the films to an atmosphere maintained at100° F and 95% relative humidity. Measurements of the moisturetransmitted through the film samples were made after a 24 hour periodand, in the following tables and accompanying charts, are given in gramsper square meter per 24 hours per mil (g/m² /24 hrs./mil).

                  TABLE I                                                         ______________________________________                                                        Film Sample                                                   Formulation       A        B        C                                         ______________________________________                                        Polyvinyl chloride resin                                                                        100      100      100                                       Diamond 450 (in pounds)                                                       Organotin Heat    1.5      1.5      1.5                                       Stabilizer                                                                    Dioctyl Phthalate 10       2        4                                         (Primary Plasticizer)                                                         "Paraplex" G-62   5        5        5                                         (Secondary Plasticizer)                                                       Extrusion Lubricant                                                                             0.15     0.15     0.15                                      Paraffin Wax      --       --       --                                        Sunwax 4412                                                                   MVTR              44.5     72.2     88.9                                      As extruded                                                                   Annealed          38.5     62.5     109.0                                     ______________________________________                                    

                                      TABLE II                                    __________________________________________________________________________                   Film Sample                                                    Formulation    D   E   F   G   H   I   J   K   L                              __________________________________________________________________________    Polyvinyl chloride resin                                                                     100 100 100 100 100 100 100 100 100                            Diamond 450 (in pounds)                                                       Organotin Heat Stabilizer                                                                    1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5                            Dioctyl Phthalate                                                                            16  16  16  16  16  16  16  16  16                             (Primary Plasticizer)                                                         "Paraplex" G-62                                                                              5   5   5   5   5   5   5   5   5                              (Secondary Plasticizer)                                                       Extrusion Lubricant                                                                          0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                           Paraffin Wax   0.1 0.25                                                                              0.5 0.75                                                                              1.0 1.5 2.0 3.0 4.0                            Suxwax 4412                                                                   MVTR           64.0                                                                              52.9                                                                              44.8                                                                              27.0                                                                              17.2                                                                              13.7                                                                              11.1                                                                              13.9                                                                              17.2                           Annealed       53.7                                                                              31.5                                                                              10.4                                                                              5.9 4.7 4.5 3.0 1.3 2.7                            __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________                   Film Sample                                                    Formulation    M   N   O   P   Q   R                                          __________________________________________________________________________    Polyvinyl chloride resin                                                                     100 100 100 100 100 100                                        Diamond 450 (in pounds)                                                       Organotin heat stabilizer                                                                    1.5 1.5 1.5 1.5 1.5 1.5                                        Dioctyl Phthalate                                                                            5   10  15  20  30  40                                         (Primary Plasticizer)                                                         "Paraplex" G-62                                                                              5   5   5   5   5   5                                          (Secondary Plasticizer)                                                       Extrusion lubricant                                                                          0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                                                              0.15                                       Paraffin Wax   1.5 1.5 1.5 1.5 1.5 1.5                                        Sunwax 4412                                                                   MVTR                                                                          As extruded    18.7                                                                              22.4                                                                              9.7 4.0 16.6                                                                              8.6                                        Annealed       16.8                                                                              11.2                                                                              6.5 2.2 1.1 2.2                                        __________________________________________________________________________

As is apparent from Table I and the upper portion of FIG. 1, themoisture vapor transmission rates of conventional, as extruded,polyvinyl chloride films (containing no paraffin wax) are improved, yetare undesirably high, as the total plasticizer content is reduced. Someadditional improvement in this characteristic is provided by annealingof such films, except with films having relatively high plasticizercontents where annealing is actually a detriment.

Table II and FIG. 2 illustrate well the merits of the present invention.While films containing more than 4.0 parts per hundred of paraffin waxhave highly desirably moistureproofing properties, formulationscontaining such wax contents are extremely difficult to extrudecontinuously. Of particular significance is the drastic reduction inmoisture transmission rates of the films after annealing.

Table III and the graphs at the lower portion of FIG. 1 both establishthat, contrary to the data set forth in Table I, a reduction of theplasticizer content of the film of the present invention does notprovide for any improvement of the moistureproof characteristic of thefilm nor does such characteristic suffer in an annealed film having arelatively large plasticizer content.

It is to be understood that changes and variations may be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

We claim:
 1. A process for preparing a self-supporting, plasticized,uncoated polyvinyl chloride film having a high resistance to moisturetransmission comprising the steps of extruding as a film a meltcontaining a film-forming polyvinyl chloride resin and additivesincluding from about 7 to 55 parts per hundred, based upon the weight ofpolyvinyl chloride resin, of plasticizer for the film-forming polyvinylchloride resin and from about 0.5 to 4.0 parts per hundred, based uponthe weight of the polyvinyl chloride resin, of a moistureproofingparaffin wax, subsequently and while at a temperature within theorientation temperature range thereof elongating the film in at leastone of its axial directions to orient the molecules thereof andthereafter annealing the molecularly oriented film.
 2. A process asdefined in claim 1 wherein said film is annealed immediately after themolecular orientation thereof by cooling the elongated film to atemperature below the orientation temperature range thereof whileimposing tension on the elongated film to prevent any substantial changein the film dimensions, and thereafter releasing the film from theimposed tension.
 3. A process as defined in claim 1 wherein saidadditives include both primary and secondary plasticizers, said primaryand secondary plasticizers being present in a ratio ranging from 1.8/1to 3.2/1.
 4. A process as defined in claim 1 wherein said additivesfurther include from 0.5 to 4.0 parts per hundred, based upon the weightof polyvinyl chloride resin, of a paraffin wax blending agent.
 5. Aprocess as defined in claim 2 wherein said additives further includefrom 0.5 to 4.0 parts per hundred, based upon the weight of polyvinylchloride resin of a paraffin wax blending agent.
 6. A process as definedin claim 3 wherein said additives further include from 0.5 to 4.0 partsper hundred, based upon the weight of polyvinyl chloride resin of aparaffin wax blending agent.
 7. A process as defined in claim 1 whereinsaid moistureproofing paraffin wax has a melting temperature within therange of from 125° F to 200° F.